Tool steel for plastic molds

ABSTRACT

A pre-hardened and pre-tempered tool steel is used for producing molds for plastic parts, in particular matrices of large dimensions. In order to make possible high core hardness without qualitative disadvantages even at large diameters, a tool steel is proposed, which has a composition in weight-% of C=0.25 to 0.30, SI=0.04 to 0.20, Mn=1.2 to 2.0, Cr=1.0 to 2.0, Ni=0.9 to 1.5, Mo=0.3 to 0.8, V≦0.2 and Al=0.01 to 0.03, wherein the remainder consists of iron and the unavoidable impurities. The addition of boron for increasing hardness was intentionally omitted since it has been shown that with larger diameters boron results in non-reproducible hardness, in particular in the core area.

[0001] The invention relates to the use of a pre-hardened and pre-tempered tool steel for producing molds for plastic parts, in particular matrices of large dimensions.

[0002] Molds for plastics for injecting or molding plastic parts always consist of two halves, a matrix and a core element. The surface quality of the plastic part is determined to a considerable degree by the quality of the matrix material.

[0003] Requirements made on the matrix material are:

[0004] uniformly high hardness for avoiding mechanical damage and plastic deformation by scratching, wear in the course of processing the plastic, deformation in the course of moving the mold halves together, warping and crack formation under operational conditions,

[0005] uniform texture of the hardened and tempered structure and high degree of purity for assuring satisfactory machine processability and satisfactory polishing and photo-chemical graining properties.

[0006] In this case the service hardness of the mold steel of approximately 30 to 40 HRC is already set by the steel producers through hardening and tempering the rolled or forged blank (pre-hardened and pre-tempered steel for plastic molds).

[0007] Molds for processing plastic are nowadays predominantly made of steel materials nos. 1.2311 and 1.2738, as well as of AISI P 20 (see Table 1 for chemical analyses).

[0008] Moreover, for special application purposes, such as processing of wear-promoting reinforced plastic, or materials which split off hydrochloric acid, such as PVC, special exceptionally hard (50 to 60 HRC) or corrosion-resistant chromium-alloy tool steel alloys are used.

[0009] When using conventional pre-hardened and pre-tempered steel for plastic molds, a special problem lies in the limited hardenability of the steel. The requirement for even hardness throughout the entire volume of the steel mold requires through-hardening down to the core of the steel block, regardless of its dimensions.

[0010] The hardenability of the standard steel types AISI P20, 1.2311 and 1.2738 which, as a rule, are oil-hardened and tempered to approximately 30 HRC, is represented in FIG. 1. The maximum dimensions in connection with the use of the respective steel, up to which no significant reduction of the hardness in the core, compared with the edge hardness, does yet occur, can also be seen there.

[0011] The trend for increasingly large parts in the plastics industry continuously requires larger molds and steel blocks of corresponding size with dimensions of more than 1000 mm thickness. Block weights of >50 tons are customary these days in connection with these dimensions. The large forged blocks of weights of more than 80 tons, which are necessarily being used, increase segregation-caused inhomogeneities, which are further increased by increasing alloy contents and which can lead to quality problems in connection with molds for plastics (mechanical processing, polishing, graining).

[0012] For example, in EP 0 709 481 B1 a steel for the use under discussion is proposed, which contains defined amounts of boron for increasing the hardenability, wherein aluminum and titanium must be added for shielding the boron from the nitrogen. However, with increased diameters, the use of boron for improving the hardenability leads to results which cannot be reproduced, i.e. undefined hardness, in particular in the core area.

[0013] DE 195 33 229 C1 describes the use of a similar steel for gas pressure vessels, DE 37 28 476 C1 for producing pipes for door reeinforcement.

[0014] A tool steel for employment in the construction of molds for plastics is described in EP 0 431 557 B1 which, in view of satisfactory weldability, is intended to have a sulfur content of between 0.025 to 0.10 weight-%. The tool steel furthermore is intended to meet an additional requirement BH=326.0+847.3(C %)+18.3(Si %)−8.8 (Mn %)−12.5(Cr %)≧460, which means a carbon content of at most 0.208 weight-% under any circumstances. Although such a low carbon content is advantageous for weldability, it is an obstacle to achieving the required hardness of the tool steel after pre-hardening and pre-tempering.

[0015] The object of the invention is to create a tool steel for molds for plastics which allows high core hardness without qualitative disadvantages even at large diameters.

[0016] In accordance with the invention, this object is attained by the use of tool steel having the following composition (in weight-%):

[0017] C=0.25 to 0.30

[0018] Si=0.04 to 0.20

[0019] Mn=1.2 to 2.0

[0020] Cr=1.0 to 2.0

[0021] Ni=0.9 to 1.5

[0022] Mo=0.3 to 0.8

[0023] V≦0.2

[0024] Al=0.01 to 0.03

[0025] wherein the remainder consists of iron and the unavoidable impurities.

[0026] For avoiding qualitative disadvantages, the tool steel in accordance with the invention is purposely produced without the addition of boron, wherein the required hardness is achieved by the specifically calculated nickel content. It has been surprisingly found that with carefully set amounts of nickel the lack of boron, which is only present in connection with unavoidable impurities, does not result in disadvantages regarding the hardness values which can be attained, while at the same time the steel displays results in connection with its hardenability, which can be better reproduced, along with a reduced tendency to embrittlement, in particular in connection with workpiece diameters of increased size. Accordingly it is also not necessary to add titanium for shielding the boron against nitrogen.

[0027] Thus, the tool steel in accordance with the invention is ideally suited for use in molds for plastics, in particular also with increased diameters of the workpiece of more than 400 mm up to 1300 mm and more.

[0028] Moreover, an improved tempering strength is achieved by means of the weight proportion of molybdenum between 0.3% and 0.8%, preferably between 0.4% and 0.6%, provided in accordance with the invention, so that with a workpiece diameter of 1000 mm, for example, and a content of Mo of 0.5 weight-%, for example, a tempering temperature of approximately 620° C. is advantageous for achieving 30 HRC, while known tool steel is hardened and tempered at tempering temperatures clearly below 600° C.

[0029] The mold steel in accordance with the invention can be hardened in water or polymer without the danger of tension cracks, has only slight segregations and a high degree of purity and, because of the increased tempering temperature, is very homogeneous and low in inherent stresses following the heat treatment.

[0030] To improve weldability, it is possible to provide a sulfur content of up to 0.15 weight-%, however, as a rule, for assuring a high degree of purity it will be attempted to keep the sulfur content, and also the phosphorus content, as low as possible, for example S≦0.005% and/or P≦0.02%.

[0031] By way of example, mold steel produced within the scope of the composition in accordance with the invention is compared with commercially customary steel in Table 1 and FIG. 1. In this case the carbon was reduced sufficiently so that segregations were minimized and hardenability in water is provided at the same time (C max. 0.30%). For minimizing block segregations, the silicon content was lowered to an average value of 0.1%, and the chromium content from 2.0 to 1.25%. The hardenability-increasing alloy content of Mn (approximately 1.50%) has remained unchanged to a large degree, while the Ni content (approximately 1.10%), which also corresponds to the steel 1.2738, together with the other contents results in the surprisingly satisfactory overall properties.

[0032] Because of the hardening-increasing effects, which cannot be reproduced in connection with large dimensions, and the simultaneous embrittling properties, the addition of boron was intentionally omitted, therefore the addition of titanium can also be omitted.

[0033] The improved tempering strength is set by means of higher amounts of molybdenum (0.5% instead of approximately 0.20% with 1.2738) and vanadium (0.1% in comparison with no vanadium with 1.2738). In this case, the tempering temperatures in connection with large dimensions of more than 1000 mm for approximately 30 HRC change as follows: Mold steel 1.2738 following oil hardening: 595° C. Mold steel in accordance with the invention: 620° C.

[0034] The sulfur content is limited to approximately 0.002% for assuring a high degree of purity.

[0035] A comparison of the hardenability of the mold steel in accordance with the invention with commercially customary standard steel is represented in FIG. 1. In accordance with this, the steel is suitable without qualitative disadvantages for pre-hardened and pre-tempered sizes of up to 1300 mm. 

1. Use of a tool steel for producing molds for plastic parts, having the following composition (in weight-%): C=0.25 to 0.30 Si=0.04 to 0.20 Mn=1.2 to 2.0 Cr=1.0 to 2.0 Ni=0.9 to 1.5 Mo=0.3 to 0.8 V≦0.2 Al=0.01 to 0.03 wherein the remainder consists of iron and the unavoidable impurities.
 2. Use of a tool steel in accordance with claim 1, characterized by an Ni content between 0.9 and 1.2 weight-%.
 3. Use of a tool steel in accordance with claim 1 or 2, characterized by an Si content between 0.004 and 0.15 weight-%.
 4. Use of a tool steel in accordance with one of the preceding claims, characterized by an Mn content between 1.2 and 1.6 weight-%, an Mo content between 0.4 and 0.6 weight-%, and a V contents between 0.05 and 0.15 weight-%.
 5. Use of a tool steel in accordance with one of the preceding claims, characterized by an S content between 0.008 and 0.15 weight-%.
 6. Use of a tool steel in accordance with one of the preceding claims, characterized in that it is water- or polymer-hardened and tempered and has a maximum hardness of 40 HRC.
 7. Use of a tool steel in accordance with one of the preceding claims, characterized in that, following hardening, it is tempered at a tempering temperature of approximately 620° C. for approximately 30 HRC.
 8. Use of a tool steel in accordance with one of the preceding claims, characterized in that it is pre-hardened and pre-tempered with a diameter of at least 400 mm.
 9. Use of a tool steel in accordance with claim 8, characterized in that it is pre-hardened and pre-tempered with a diameter of at least 1000 mm. 